1. Field of the Invention
The present invention relates to a lens array of an erecting mode with unity amplification and, more particularly, to a lens array of an erecting mode with unity amplification, which provides an aperture diaphragm and prevents a flare light and a stray light.
2. Description of the Prior Art
A lens array of an erecting mode with unity amplification is realized by superposing a plurality of lens plates, which array a number of convex lenses on both sides thereof. For example, a plurality of resin lens plates, which array a number of convex lenses on both sides of a transparent substrate, is superposed so as to form a resin lens array of the erecting mode with unity magnification.
An optical system of the erecting mode with unity magnification assumes that the height of an object is equal to the height of an erecting image. It is, thus, subject to the condition that the optical system between the object and the erecting image is symmetric, and an inverted image needs to be formed in the center of a lens plate group. In the case where the number of lenses is in even numbers, the inverted image is formed between two pieces of the lens plates at the center, and in the case where the number of lenses is in odd numbers, at the central position of the lens plate at the center, and a light ray becomes symmetrical for this inverted image.
FIG. 1 shows a state of the light ray in which an image of the erecting mode with unity magnification is obtained by tightly superposing two pieces of the lens plates 10 and 12 in which convex lenses 8 are arrayed and formed on both sides. In the drawing, reference numeral 2 denotes the object, and 4 the Image. In this case, the inverted image 16 is formed on the surface 14 to which the lens plates 10 and 12 come into contact.
Since the lens plate can be manufactured by a forming, a mass production Is easy and, since the lens plate is light in weight and moderate in price, it is used for various applications. In particular, applications for image forming apparatuses such as optical printers and the like or image reading apparatuses such as scanners and the like are expected. Although a much higher resolution is required for these apparatuses, the resin lens by a forming can obtain a high degree of accuracy since an accuracy of the lens array is decided by the accuracy of a forming die. Moreover, the resin lens by forming is characterized by the very rare existence of characteristic unevenness between individual lens arrays.
The resin lens array of the erecting mode with unity magnification as described above has the following problems.
(1) Since the lens plate is transparent, the portion other than the lens portion is transparent. Hence, a light shielding is required for the light that transmits the portion other than the lens portion, and a shielding layer is thus provided. However, even if the shielding layer is provided, there is such a case that the light incident on the lens portion becomes the flare light and the stray light for adjacent lenses.
(2) When the area of the shielding layer is enlarged to prevent the flare light and the stray light, the amount of transmitted light is reduced.
The above-described problems will be described more in detail in relation to a compact lens array structure. As an ordinary compact lens array structure, there exist a six directional compact array structure and a four directional compact array structure. FIG. 2A shows one piece of a lens 20 in the case of the six directional compact lens arrays. Although the actual lens shape of this lens is an equilateral hexagon, the lens diameter can be regarded as the diameter of an inscribed circle 22 of the equilateral hexagon. FIG. 2B shows one piece of a lens 24 in the case of the four directional compact array structures. The actual lens shape of this lens is a square, and the lens diameter can be regarded as the diameter of an inscribed circle 26 of the square.
Continuing with FIGS. 1 and 2A and 2B, the inverted image 16 needs to be formed within the scope of the inscribed circles 22 and 26. The height of the inverted image is theoretically found to satisfy a required specification of a desired resolution and the amount of transmitted light. In this way, the size of the actual lens is designed. Although the height of the inverted image changes depending on the design of the optical system, the maximum value of the height of the inverted image needs to be within a lens radius (radius of the inscribed circle of the actual lens shape). When the height of the inverted image becomes higher than the lens radius, the part of the inverted image hangs over the adjacent lenses so that no correct image formation can be made.
It is possible to design the optical system so that the height of the inverted image becomes lower than the lens radius. In general, the outer periphery of the lens has a large aberration and, therefore, It is desirable to design the height of the inverted image slightly lower than the lens radius. However, when the height of the inverted image is low, the amount of transmitted light is reduced and the image becomes dark. Therefore, it is necessary to design the optical system by considering a balance between both factors. On the other hand, the height of an object 2 needs not to be equal to the lens radius, and if it is within the scope in which the light can be taken in, regardless of whether it is higher or lower than the lens radius, it can be selected by the design of the optical system.
Returning to FIGS. 2A and 2B, the light which passes through within the real lens region of the outside of the inscribed circles 22 and 26 becomes the flare light. Further, the light which passes through within a virtual lens radius equivalent to the radius of the inscribed circles 28 and 30 of the real lens becomes the stray light at the outside of the real lens region.
FIGS. 3A and 3B show the lens plates, which are constituted by a six directional compact array, or a four directional compact array in which such real lenses mutually come into contact. Note that the array direction of the real lens is defined as follows. That is, the direction, in which one side of the shape of a polygonal real lens comes into contact with each other, is regarded as the array direction. Hence, it is clear that there are the six directional array directions in FIG. 3A, and the four directional array directions in FIG. 3B.
In the lens array of the erecting mode with unity amplification constituted by superposing a plurality of lens plates shown in FIGS. 3A and 3B, when the inverted image formed by a pair of lens enters the real lens region of the outside of the inscribed circle, it becomes the flare light. When it enters within the virtual lens radius outside of the real lens region, it becomes the stray light. To prevent these phenomena, when the shielding layer is provided in the boundary of the real lens so as to form the aperture diaphragm therein, the lens area, through which the inverted image can transmit, is decreased and the amount of transmitted light is reduced. Further, when there is unevenness in the width or the position of the shielding layer, it tends to cause an uneven light intensity. The image forming apparatus or the image reading apparatus using such a lens array of the erecting mode with unity magnification generate the uneven light intensity due to the flare light and the stray light.
An object of the present invention is to provide a lens array of an erecting mode with unity amplification, which scarcely generates a stray light and a flare light.
In the lens array of the erecting mode with unity magnification of the present invention, which is constituted by superposing a plurality of lens plates in which convex lenses are arrayed and formed on both surfaces, a lens pitch in the array direction of the convex lens is not less than two times the height of the inverted image formed inside the lens array of the erecting mode with unity amplification, and an aperture diaphragm is provided on individual lens elements in the surface adjacent to the position in which the inverted image is formed, so that a light does not pass through the outside of the area of a circle having the height of the inverted image as a radius. In this case, when the number of the lens plates is in even numbers, the aperture diaphragm is provided between two pieces of the lens plates in which the inverted image is formed.
Further, when the number of lens plates is in odd numbers, the aperture diaphragm is provided inside the central lens plate, inside of which the inverted image is formed. The central lens plate is constituted such that two pieces of single-faced lens plates, in which convex lenses are formed on one side, allow the surfaces, in which no convex lenses are formed, to be opposed and superposed, and the aperture diaphragm is provided on the superposed surfaces of two pieces of the single-faced lens plates. Further, when the number of lens plates is in odd numbers, the aperture diaphragm is provided between the central lens plate, inside of which the inverted image is formed, and the lens plates of both sides in opposition to the central lens plate.
The aperture diaphragm can be formed of a film-shielding layer or the shielding layer adhered to the surface of the lens plate.